9 Elementary Biology. 



either it must be poisoned by its own excreta, or it must 

 divide into two or more parts, so as to restore the proper 

 proportion of surface and mass. Hence at the limit of 

 growth, in Protococcus for example, division takes place. 

 This division amongst the lowest types represents asexual 

 multiplication. 



It is conceivable, however, that the division might result 

 in the formation of two cells not equally anabolic or kata- 

 bolic. For instance, one half might contain an excess of 

 protoplasm which had a tendency to break down, or exhibit 

 energy in movement ; the other half might be more ana- 

 bolic, passive, and sluggish. We should have thus formed 

 an incipient sexual differentiation ; the more katabolic cell 

 would represent the male cell, or sperm, whilst the more 

 anabolic would represent the female cell, or ovum. Atten- 

 tion was directed in the case of Spirogyra to the difference 

 in physiological activity observed between the two conju- 

 gating cells, and it was there pointed out that it was pro- 

 bable that the more kinetic cell represented the male and 

 the more sluggish the female element in true sexual conju- 

 gation. 



The kinetic or katabolic cell may be looked upon as the 

 liberating energy which sets off, so to speak, the potential 

 energy, just as the spark sets off the gunpowder; only in this 

 case the conditions are more complicated. The kinetic 

 energy must be energy resulting from the decomposition of 

 special chemical compounds, and the products of kata- 

 bolism must themselves be such as are able to stimulate the 

 ovum to undergo those changes which convert it into an 

 embryo. This subject will be referred to again when we 

 come to consider the fertilisation of the higher plants and 

 animals. 



Amongst salt-water Algse, as amongst fresh- water forms, 

 we meet with great variety in the form and structure of the 

 thallus. All agreeing in the general characters already de- 

 scribed in the type discussed above, they yet present almost 



